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Extreme morning sickness blighted both of the Duchess of Cambridge's pregnancies - raising the chances that sooner or later she would give birth to a girl, according to scientists.

Researchers who looked at 1.7million pregnancies found women who suffer from the condition are more likely to have a princess of their own - rather than a boy.

Their analysis linked the condition, known as hyperemesis gravidarum, with unusually low odds of having a son.

It is unclear why but one possibility is that male babies may be more fragile in the womb and so less able to withstand a difficult pregnancy. Much more severe than normal morning sickness, hyperemesis gravidarum strikes in one in 100 pregnancies.

The Duchess of Cambridge was so badly ill during her first pregnancy with Prince George that she spent three days at the King Edward VII private hospital in Central London in December 2012. Affected again while carrying Princess Charlotte, she received treatment at her Kensington Palace home.

Generally, slightly more boys are born than girls. For instance, in the UK around 105 boys are born for every 100 girls.

However, 56 per cent of the babies born in the Swedish data were female, New Scientist reports. The research also confirmed that women who suffer the condition are more likely to miscarry.

The researchers, from Columbia University in New York, said one explanation is that male babies may be more likely to be miscarried. Another possible explanation is female babies may simply be more likely to trigger extreme morning sickness in the first place.

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The study is far from the first to find that a woman's odds of having a boy or a girl are skewed by external events.

Previous research has found that women who were under pressure at home, work or in their love life in the weeks or months before becoming pregnant had higher than usual odds of giving birth to a daughter rather than a son.

It has also been shown that even royal events affect the gender of newborns, with a study finding a striking dip in the number of boys born around the time of Charles and Diana's 1981 wedding.